1. Field of the Invention
The present invention relates to a cooling control system for cooling an internal combustion engine such as an engine for an automobile, and particularly to a cooling control system for an internal combustion engine, which can prevent the engine from overheating in the event that a failure occurs in a control system controlling the flow of a cooling medium, or the like.
2. Description of the Related Art
In an internal combustion engine (hereinafter referred to as an engine) which is used for an automobile or the like, a water cooling type of cooling apparatus using a radiator is generally used for cooling the engine.
In this type of cooling apparatus, a thermostat for controlling the temperature of cooling water is used, and when the temperature of the cooling water is lower than a specified temperature, the cooling water is passed through a bypass conduit and is circulated without passing through a radiator under the control of the aforesaid thermostat.
FIG. 8 illustrates the configuration, and a numeral 1 represents an engine composed of a cylinder block 1a and a cylinder head 1b. Fluid conduits shown by arrows c are formed in the cylinder block 1a and the cylinder head 1b of the engine 1.
A numeral 2 represents a heat exchanger, that is, a radiator, and a fluid conduit 2c is formed in the radiator 2 as is known. A cooling-water inlet portion 2a and a cooling-water outlet portion 2b of the radiator 2 are connected to a cooling-water conduit 3 for circulating cooling water between the aforesaid engine 1 and the radiator 2.
The cooling-water conduit 3 is composed of an outflow-side cooling-water conduit 3a, which is communicated with a cooling-water outflow portion 1d provided at the upper portion of the engine 1 and the cooling-water inflow portion 2a provided at the upper portion of the radiator 2, an inflow-side cooling-water conduit 3b, which is communicated with the cooling-water outflow portion 2b provided at the lower portion of the radiator 2 and a cooling-water inflow portion 1e provided at the lower portion of the engine 1, and a bypass conduit 3c connecting the portion between both of the coolingwater conduits 3a and 3b.
A thermostat 4 is disposed at the portion branching into the outflow-side cooling-water conduit 3a and the bypass conduit 3c in the cooling-water conduit 3. The thermostat 4 incorporates a thermal expansive body (for example, wax) which expands or shrinks depending on a change in the temperature of the cooling water. The thermostat 4 has the following operation: when the temperature of the cooling water is higher (for example, more than 80.degree. C.), the thermostat 4 opens a valve by the expansion of the aforesaid thermal expansive body, and allows the cooling water flowing out of the outflow portion 1d of the engine 1 to enter the radiator through the outflow-side cooling-water conduit 3a, then allowing the cooling water, having lower temperature as a result of the heat radiation conducted in the radiator 2, to flow out of the outflow portion 2b to pass through the inflow-side cooling-water conduit 3b and to flow into the engine 1 from the inflow portion 1e of the engine 1.
When the temperature of the cooling water is lower, the valve of the thermostat 4 is closed as a result of the thermal expansion body shrinking, and the cooling water flowing out of the outflow portion 1d of the engine 1 is designed to pass through the bypass conduit 3a and to flow into the cooling conduit c in the engine 1 from the inflow portion 1e of the engine 1.
It should be mentioned that a numeral 5 in FIG. 8 represents a water pump disposed at the inflow portion 1e of the engine 1, which compulsorily circulates the cooling water, with its rotational shaft being rotated by the rotation of a crankshaft, not illustrated, of the engine 1. A numeral 6 is a fan unit for compulsorily taking cooling air into the radiator 2, and the fan unit 6 is composed of a cooling fan 6a and a fan motor 6b for rotationally driving the cooling fan 6a.
The valve opening and closing operations by the thermostat as described above is determined by the temperature of the cooling water, and the operation is made as a result of the expanding or the shrinking action of the thermal expansive body such as wax, therefore the temperatures for opening and closing the valve are not fixed. Specifically, it takes some time for the thermal expansive body such as wax to operate the valve after the thermal expansive body is given a change in the temperature of the cooling water. The responsiveness to a decrease in temperature is especially worse compared to that to an increase in temperature, and has so-called hysteresis properties. For this reason, there is a technical disadvantage of extreme difficulty in regulating cooling water at a desired fixed temperature.
Hence, an apparatus for electrically controlling the flow of cooling water without utilizing the expansion of a thermal expansive body such as wax for opening and closing operations of a valve is proposed.
The apparatus controls the rotational angle of a butterfly valve by means of a motor. The thermostat 4 in FIG. 8 is omitted, and a valve unit 7 is disposed at the outflow-side cooling-water conduit 3a equipped with the butterfly valve instead of the thermostat 4 as shown by a broken line in FIG. 8.
FIG. 9 shows an example of the valve unit 7, and a butterfly valve 7a in a circular flat plate shape is rotatably supported by a shaft 7b inside the cooling-water conduit 3a. A worm wheel 7c is attached at one end of the shaft 7b, and a worm 7e fitted in the rotational driving shaft of a motor 7d is meshed with the aforesaid worm wheel 7c.
The aforesaid motor 7d is supplied with operating current, which normally rotates and reverses the driving shaft thereof, by a control unit (ECU) controlling the driving state of the entire engine. Accordingly, when the current for normally rotating the driving shaft is supplied to the motor 7d by the operation of the ECU, the shaft 7b of the butterfly valve 7a is rotated in one direction by the known deceleration action of the worm 7e and the worm wheel 7c, thereby the butterfly valve 7a is rotated so that the face thereof is in the same direction as the flowing direction of the cooling-water conduit 3a to establish a valve opened position.
When the current for reversing the driving shaft is supplied to the motor 7d by the operation of the ECU, the shaft 7b of the butterfly valve 7a is rotated in another direction, thereby the butterfly valve 7a is rotated so that the face thereof is in a direction perpendicular to the direction of the cooling-water conduit 3a to establish a valve closed position.
The aforesaid ECU is supplied with information regarding, for example, the temperature of the cooling water of an engine, and the temperature of the cooling water can be controlled by controlling the aforesaid motor by utilizing the information.
However, in the cooling control system using the aforesaid butterfly valve, the opening and closing operations of the butterfly valve can't be made in a case where, for example, a breakdown occurs in the motor, or a failure occurs in the worm gear portion.
When the aforesaid breakdown or failure occurs, for example, when the butterfly valve is in the valve closed position, or in a valve-partially-opened position, sufficient cooling operation of the engine is not carried out, and therefore there is a technical disadvantage of the engine overheating while a driver does not notice that.
In order to avoid the above disadvantage, a mechanism directly driving the butterfly valve without using the aforesaid worm gear is conceivable, and it is further conceivable to provide a return spring for giving the momentum to drive the butterfly valve to the valve opened position. In this configuration, the butterfly valve can be automatically opened by the momentum given by the return spring even when a failure occurs, thereby preventing the engine from overheating.
However, when driving a butterfly valve, 0.5 Kg.multidot.cm is needed as a friction of the butterfly valve, 2.0 Kg.multidot.cm is needed as torque of the valve against the water pressure of cooling water, and 2.5 Kg.multidot.cm is needed as torque against the return spring.
Accordingly, in order to drive the butterfly valve, torque of more than 5.0 Kg.multidot.cm is needed. Consequently, an actuator such as a motor or linear solenoid for giving the above driving force is inevitably larger in size, therefore there is a disadvantage of a greater volume it constitutes.
In addition, the aforesaid configuration, in which the butterfly valve is directly driven by the actuator, employs a driving method for balancing the valve opening position with the momentum given by the return spring and the driving force from the actuator driving the butterfly valve when holding the butterfly valve at a certain fixed rotational angle, therefore there is a disadvantage of a necessity arising to always supply driving current to the aforesaid actuator.